1. Field of the Invention
This invention relates to a solder paste suitable for use in a reducing atmosphere. More particularly, the solder paste includes a solder powder and a binder comprising alcohol.
2. Description of the Related Art
Solder is frequently used in the production of electronic components to join integrated circuit modules or chip carriers to circuit cards or circuit boards. For example, solder may be used to connect conductive metal pins from a module to the conductive circuit lines of a circuit card. It is known to apply solder to the components in the form of a solder preform or a solder paste. A solder preform is a solid composition of solder fabricated to the shape and dimensions required to contact the desired locations of each of the components to be joined. The preform is placed in proper position and subsequently heated to cause the solder to flow, or "reflow," and physically join the components. A solder paste is a composition of a solder powder in one or more liquid solvents or binders. The paste is screened on to the components, dried, and heated to reflow the solder and join the two components. For both preforms and pastes, a liquid flux is typically used to deoxidize the metal surfaces of the components to cause them to accept the solder.
The use of a conventional liquid flux results in flux residues left behind on the surface of the components after soldering. For example, a common flux such as waterwhite rosin leaves a metal salt residue abieate formed when the abietic acid in the rosin reacts with oxides on the metal surfaces of the components. Where the residue contacts the metal surfaces of the components, it will cause detrimental galvanic corrosion upon the passage of electrical current in normal use. Thus, the residue must be removed from the components after soldering and before electrical use.
Several problems are associated with the removal of flux residue. The first problem is the cost of solvents used to remove the residue, such as perchloroethylene or N-methyl pyrollidone, Methylene Chloride, Methylchloroform, or Freon. In addition, costly scrubber systems are required to reduce hazardous emissions of these solvents into the atmosphere. Finally, some hazardous emissions still occur despite the scrubbers, resulting in intangible harm to the environment.
One soldering process is known to eliminate flux residues. To prevent flux residue, the process replaced conventional liquid flux with a reducing atmosphere. By heating a solder preform in a reducing atmosphere such as hydrogen or carbon monoxide instead of an inert atmosphere such as nitrogen, oxides are removed from the metal surfaces of the components. The need for liquid flux is therefore eliminated. In addition, the "fluxless" process (i.e., no liquid flux is used) by-products are gases and certain hydrocarbons which can be readily removed by burning off with excess hydrogen to form carbon dioxide and water vapor.
Conventional soldering processes have not eliminated the problem of solder splattering during reflow. Splattered solder causes shorting between the individual conductive circuit lines of the components. In conventional processes, the liquid rosin flux breaks down when the solder is heated. The aforementioned reaction between metal oxide and abietic acid produces gases such as carbon dioxide and water vapor. These by-products outgas from the molten solder, causing such to splatter upon the components.
A final problem associated with that of soldering electronic components is that of precision. To assure that proper electrical connections, electrical components must often be joined according to tight dimensional tolerances. Solder preforms are inherently difficult to use in such applications because precision is limited by the accuracy by which the preform is placed upon the components. Hence, solder pastes are the preferred form for handling solder in such applications.